1. Statement of the Technical Field
The inventive arrangements relate to inductors on semiconductor substrates and methods for forming the same, and more specifically to spiral type inductors on semiconductor substrates having a high quality factor.
2. Description of the Related Art
Spiral inductors are commonly used in monolithic integrated circuits designed for radio frequency (RF) applications. For example, spiral inductors are frequently formed on semiconductors substrates comprised of silicon or gallium arsenide.
Conventional spiral inductors in silicon-based integrated circuits typically have performance limits with respect to the quality factor (Q factor) and the self resonant frequency (the maximum frequency at which the inductor behaves ideally). This limited performance is due to a variety of factors including the series resistance RS representing the ohmic losses in the coil, and substrate losses. Substrate losses are usually associated with the oxide capacitance COx, and bulk silicon substrate resistivity losses RB. In general, these factors tend to degrade the Q factor and lower the self-resonant frequency of the inductor.
Some efforts to improve Q have focused on reducing ohmic losses in the coil. This has been done by fabricating the basic coil using several metal layers that are shunted together using multilevel interconnects. Some designs have also chosen to omit the lowest metal layers to reduce COx. In other design, selective removal of portions of the substrate beneath the coil has been shown to further improve the value of Q. Still, these efforts have not achieved an entirely satisfactory level of Q and/or self resonance frequency. Improving the Q and/or the self resonant frequency would increase the usefulness of the inductor in many applications.